Sample introduction systems have been the weak link of ICP spectrometry. In ICP spectrometry, a liquid sample solution typically is converted into a form of aerosol, carried by an inert gas such as argon, and then injected into an ICP spectrometer for analysis. The converting of the liquid sample is normally accomplished through use of a nebulizer. Prior to the aerosol's being injected into the ICP spectrometer, large droplets of sample are removed by means of a spray chamber, for example. Only the smaller, useful particles are introduced into the ICP spectrometer. "Nebulization efficiency" is a relevant factor in this process and is defined as the amount of sample introduced into the ICP spectrometer after the removal of large droplets (i.e., useful aerosol) divided by the total amount of sample initially delivered to the nebulizer. Problems associate with low nebulization efficiency and current devices, which are generally of low nebulization efficiency, are unsuitable for analysis of samples introduced over a wide range of flow rates, and often require the use of a high pressure pump or other mechanism to introduce an adequate volume of sample for experimental purposes. In general, there are two types of sample introduction systems, ultrasonic nebulizers and pneumatic nebulizers. Less common devices include thermospray and direct injection nebulizers. A brief discussion of various relevant nebulizers is presented below. A more detailed and thorough discussion of various nebulizers and of nebulization generally is presented in the specification and drawings of U.S. Pat. No. 5,411,208 issued to John A. Burgener on May 2, 1995 and the text and drawings of that patent are incorporated herein by reference for purposes of clarification where necessary.
Ultrasonic nebulizers typically offer 10 times the nebulization efficiency of pneumatic nebulizers. However, ultrasonic nebulizers are more complicated to operate than pneumatic nebulizers. Also there have been reports regarding interferences due to nebulization desolvation for the ultrasonic nebulizer. "Nebulization desolvation" is a process used to remove water vapor and large particles prior to aerosol injection into the ICP spectrometer. Less complicated pneumatic nebulizers can be classified as cross-flow and concentric nebulizers. Among all sample introduction systems, glass concentric nebulizers are the most popular due to their simplicity in design and operation. The current invention relates closely with glass concentric nebulizers.
The basic operating principle of a glass concentric nebulizer is simple and is explained with the aid of FIGS. 1 to 3. FIG. 1 depicts the structure of a typical glass concentric nebulizer. The nebulizer is a single piece formed from glass. The nebulizer has an elongated hollow main tube with a rear end and a front end. The front end is tapered down in roughly the shape of a cone and terminates in an opening. Coming off the side of the main tube is a gas tube for feeding gas into the main tube for expulsion through the opening in the front end of the main tube. The main tube carries in its interior an integrally formed central capillary which extends from the rear end of the main tube to the front open end of the main tube and is aligned in concentric fashion with respect the main tube. The outside of the central capillary and the inside of the rear end of the main tube are sealed together as shown in FIG. 1 so that gas being fed into the main tube from the gas tube cannot escape through the rear end of the main tube, but instead, is forced to escape through the opening in the front end of the main tube. The inner diameter of the main tube is larger at all points along its length than the outer diameter of the central capillary so that gas can escape through the space between the two at the front end of the main tube. The space between the capillary and the front end of the main tube is referred to as the gas annulus.
In operation, a liquid sample is fed into the rearward end of the central capillary and is expelled through the forward end of the capillary. At the same time, gas is fed into the main tube from the gas tube. The liquid sample may be moved through the central capillary by free suction created by the partial vacuum at the forward end of the capillary due to the rapidly exiting gas or by pumping it into the rearward end of the capillary or both. The sample flow rate created by free suction at given gas flow rates is known as the "natural aspiration rate." As the liquid exits the capillary, it interacts with the gas being expelled under pressure through the gas annulus in the main tube and forms an aerosol. Typically, inert gases such as argon are used, but any gas may be used that is consistent with protocol for a particular experiment. As can be seen in FIG. 1, the forward end of the capillary cooperates with the opening in the main tube to form a nozzle. The translational position of the capillary's forward end with respect to the opening in the main tube is critical in aerosol formation. There are typically three configurations for these types of nozzles. In one configuration, the capillary's forward end extends outside the glass tube, beyond the opening in the main tube. In a second configuration, the capillary's forward end is flush with the opening in the glass tube. In a third configuration, the forward end of the capillary is recessed with respect to the opening in the glass tube. It is easy to appreciate based on how these nebulizers are manufactured that the nozzle configuration is fixed permanently for any single glass concentric nebulizer. Not only is the position of the forward end of the capillary fixed with respect to the opening in the front of the main tube, but other parameters critical to aerosol formation are fixed as well such as the inner and outer diameters of the central capillary, the inner diameter of the main glass tube at its front end opening, the size of the opening at the front end of the main tube, and the cross-sectional area of the gas annulus. All of these parameters play a central role in the formation of aerosol and, because they are fixed for any given single-piece nebulizer, frequent changing of entire nebulizers during experimentation is necessary where different analytical applications (e.g. different flow rates) are desired.
At present, there are generally two types of glass concentric nebulizers, one for regular sample flow rates and the other for micro-volume sample flow rates. "Regular sample flow rate" is on the order of milliliters of sample per minute (mL/min) flowing through the capillary while "micro-volume sample flow rate" is on the order of microliters of sample per minute (.mu.L/min) flowing through the capillary. For best results, the sample flow rate used should be close to the natural aspiration rate. If the sample flow rate is much lower than the natural aspiration rate, pulsation in nebulization occurs due to the sudden burst of sample aerosol created by free suction which is followed by a bubble at the capillary tip. This phenomenon is referred to as the "nebulization starvation effect." For example, the regular MEINHARD glass concentric nebulizer (FIG. 2) passes anywhere from 0.5 to 2 mL/min. of sample through its capillary at the natural aspiration rate and the MEINHARD glass concentric High Efficiency Nebulizer (FIG. 3) typically passes less than 100 .mu.L/min of sample through its capillary at the natural aspiration rate under typical conditions. These two nebulizers are almost identical except for the nozzle opening and the capillary. The inner diameter of the sample capillary is the primary factor that determines the sample flow rates of the two nebulizers. A regular flow rate device such as that in FIG. 2 possesses a capillary with a larger inner diameter ranging from 220-320 plus microns as compared to approximately 100 microns for the micro-volume device of FIG. 3. Because of their larger capillary inner diameters, regular concentric nebulizers are not suitable for micro-volume sample analysis due to the "nebulization starvation effect" under the typical operating conditions. Relatedly, use of the micro-volume devices at an increased sample flow rate (e.g., greater than 0.5 mL/min) is difficult because the reduced inner diameter of the capillary limits the volume of sample that can flow through the capillary per unit time. To increase the sample flow rate through these micro-volume nebulizers, a high pressure pump is required to pump the sample through the capillary. However, use of a low pressure sample pump (e.g., a peristaltic pump) is preferable in ICP spectrochemical analysis because it allows easy cleaning and rapid sample switch over.
Another difference between the regular and high efficiency nebulizers is the gas operating pressure. Because the gas annulus for the high efficiency nebulizer is typically on the order of 5 times smaller in area than that of the regular concentric nebulizer, the gas operating pressure is about 180 psi as compared to 20 to 60 psi for the regular nebulizer. The nebulizing gas flow rate for both nebulizers under normal conditions is around 1 liter per minute. Because of the small gas annulus, higher pressure is needed to force gas through the gas annulus of the high efficiency nebulizer than the gas annulus of the regular nebulizer. Again, low operating pressure is desirable for normal analytical applications.
As for sample flow rates, to pump sample through the capillary of a regular nebulizer at a rate of 1-2 mL/min, only a low pressure pump (e.g. a peristaltic pump) is required. However, for a high efficiency nebulizer, a high pressure pump is required due to the smaller inner diameter of the sample capillary.
In the cases of both gas and sample solution, low operating pressure is preferred because it makes conducting experiments simpler and safer.
An alternative to the nebulizers currently used to analyze samples at different sample flow rates is to use just one nebulizer with a replaceable capillary and a replaceable main tube with a tapered open front end. This is not possible with the glass concentric nebulizers commonly in use because the entire device is sealed by glass-blowing various components together to form a single, inseparable article of manufacture. To remove a capillary from the single piece glass concentric nebulizer would require the destruction of the entire device. Similarly, replacing other individual parts of these nebulizers, such as the gas tube, is not possible for the same obvious reason. Presented below are the details of the present invention which, among other things, permit a user to change capillaries, adjust the position of capillaries with respect to the opening at the front end of the glass tube, and interchange the functional equivalent of the main glass tubes to vary the size of the front end opening.
Other, less relevant, devices related to the field of the current invention include the micro concentric nebulizer and the oscillating concentric nebulizer.
The micro concentric nebulizer, developed at CETAC Technology, Inc. (Omaha,Nebr.), is made of various materials, PVDF(Kynar), sapphire, polyimide, PEEK, and TEFLON among them. It is also a concentric nebulizer that applies principles of pneumatic nebulization. The nebulizer is designed for low sample flow rates similar to those of the high efficiency nebulizer discussed earlier with the exception that the device can be operated at reduced gas pressure. Because the outer diameter of this nebulizer body is much greater than that of the glass concentric nebulizer, this nebulizer requires its own mount to the spray chamber of ICP. In addition, the CETAC device does not permit a user to simply interchange capillaries of different inner and outer diameters nor does it allow a user to vary the gas pressure over a continuum because it lacks a tapered front end in which the position of the capillary with respect to the front end may be adjusted.
The oscillating capillary nebulizer, developed at Georgia Institute of Technology (Atlanta, Ga.), is made by forming a nebulizer nozzle with a pair of chromatograph columns. The operating principle of this nebulizer combines pneumatic effect and center capillary oscillation. Oscillation occurs when sample and gas are introduced and is in the range of 200 Hertz to 1400 Hertz. The oscillation begins in the inner capillary which in turn induces oscillations in the outer capillary. Typical inner diameters for the inside and outside capillaries are 50 microns and 250 microns, respectively. In the device as actually constructed, the outer diameters would typically be 142 microns and 440 microns, respectively. The positions of the capillaries are fixed by using stainless steel nuts and PEEK tubing ferrules. The entire nebulizer body is constructed of stainless steel. The nozzle configuration (i.e., the position of the inner capillary with respect to the outer capillary) is adjusted through a rotating connecting ring. An O-ring seal is applied in the connection. The preferable nozzle configuration for this nebulizer is to have the inner capillary extend outside of the outer capillary. This design allows replacement of various parts including the capillary pair for the nozzle. However, because the outer capillary is too small and not as strong as the outer shell of the main tube of a glass concentric nebulizer, a special mount is also required for using the Georgia Institute of Technology nebulizer with a common spray chamber of an ICP spectrometer. In addition, because this device uses two commercially available capillaries for nozzle fabrication, the gap for the gas passage (gas annulus) between two capillaries can not be varied continuously because the capillaries have constant radii over their entire lengths. The ability to vary the ratio of the radii is necessary to obtain different nebulizing gas pressures. For example, for a given sample capillary, a continuous increase of the inner diameter of the outside capillary (or main tube) results in a gradual decrease of nebulizing gas pressure and vice versa. This adjustment capability is preferable to operate nebulizers at different gas pressures. As will be seen, the present invention achieves this feature of variable gas pressure by permitting linear movement of the central capillary within a tapered (conic) main tube front end which varies the ratio of the inner diameter of the main outer tube and the outer diameter of the central capillary.